Thanks for that - it makes sense when you look at it that way!

The process of formulating my question above set me to thinking, and I solved the problem. hdb2 being the "boot" drive, its address isn't /mnt/hdb2 - it's just /. So I checked in smb.conf, and sure enough / wasn't there as a drive to share on the network. I edited /mnt/hdb2 to say / and all was well. It was as easy as that.

Then a new problem showed up - hda1 and hdb1, which had previous been shared across the network just fine, showed in the network but refused access. Both had miraculously changed ownership from users:users to root:root. Chown refused to change them back, but a little tinkering eventually got it to see things my way. Some faults, like that one, appear for a while, then disappear for no apparent reason.